1. Field of the Invention
This invention relates, generally, to machines that apply a coating to a substrate. More particularly, it relates to a machine that applies an aqueous liquid ink or other aqueous liquid coating as a solid or patterned coverage to a continuous paper web and which dries the coating without distorting the paper.
2. Description of the Prior Art
Aqueous inks and coatings use water as the solvent with the highest concentration by volume; current technology aqueous inks and coatings contain less than five per cent (5%) by volume of organic compounds such as alcohol or alcohol substitute solvents.
The coating of paper webs at one hundred per cent (100%) areal coverage and at patterned coverages with aqueous inks or other aqueous coatings is problematic because paper absorbs water and usually stretches or curls, or both, when it dries in ambient air. Accordingly, subsequent processes applied to the web, such as those found in a printing press and various paper converting machines which assume dimensional stability of the paper, are not in register with preceding processes.
Stretching, curling, or other dimensional deformation of paper can occur as a result of the direct absorption of water and by absorption of water vapor created by the drying of water on the paper surface.
Earlier machines in this field lack the ability to apply a uniformly thin film of aqueous ink or coating across the web or in the direction of web travel to sufficiently limit direct absorption of water to a sufficiently shallow depth of penetration. They also lack the ability to rapidly evaporate water from the paper at the surface down to penetration depth while simultaneously avoiding absorption or water vapor.
Moreover, in applications which require coating both sides of the web, the earlier machines lack the ability to maintain the same coating thickness on both sides thereof.
Moreover, chemists are continually developing cleaner and safer inks and coatings which contain more water and fewer volatile organic compounds. Thus, there is a need for a machine that can apply a very thin film of aqueous ink or aqueous coating uniformly across the web, in the direction of web travel, and on both sides thereof without absorption of water. The ideal machine could even apply pure water in a uniform coating, which would assure compatibility with future cleaner and safer inks and coatings.
Paper webs can also be distorted by pulling them through a process under tension. The earlier machines also lack the ability to reinsert the web into printing presses or to drive the web into a subsequent process without tensioning the paper.
When the prior art pertaining to the field of paper web tinter-coater and drying machines was viewed as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in such art how the limitations of the earlier machines could be overcome.